1. Field of the Invention
The present invention relates to a method and related communications device for handling packets, and more particularly, to a method and related communications device for handling packets in a hybrid automatic repeat request (HARQ) process for a user end in a wireless communications system.
2. Description of the Prior Art
The third generation (3G) mobile telecommunications system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission. The WCDMA method also meets all kinds of QoS requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates. Through the 3G mobile telecommunications system, a user can utilize a wireless communications device, such as a mobile phone, to realize real-time video communications, conference calls, real-time games, online music broadcasts, and email sending/receiving. However, these functions rely on fast, instantaneous transmission. Thus, targeting third generation mobile telecommunication technology, the prior art provides High Speed Downlink Package Access (HSDPA) and High Speed Uplink Package Access (HSUPA), which are used to increase bandwidth utility rate and package data processing efficiency to improve uplink/downlink transmission rate.
In the prior art, HSDPA and HSUPA adopt Hybrid Automatic Repeat Request (HARQ) technology to enhance retransmission rate and reduce transmission delay. HARQ is a technology combining Feed-forward Error Correction (FEC) and ARQ methods, which triggers retransmission according to positive/negative acknowledgement signals (ACK/NACK) transmitted from a receiving end, and stores useful information about the last failed transmission for later use. During retransmission, the HARQ process uses the same transport block size as that used in the previous transmission, thus the retransmitted transport block has the same amount of bits. However, modulation type, channelization code set, and transmission power utilized may be different for the two transmissions. In order to minimize the number of retransmission requests, HARQ uses one of two Soft-Combining methods, namely Chase Combining (CC) and Incremental Redundancy (IR), to ensure that messages can be successfully decoded. CC transmits a packet the same as the erroneous packet, and the decoder combines all the received packets before performing decoding. IR increases redundancy to help in recovery from transmission errors, sequentially transmitting packets having parts of the erroneous packet, and the decoder combines the packets.
Through an HARQ process in HSDPA, the user equipment (UE) receives packet data units (PDUs) from a network end (UTRAN) and feeds back an ACK or a NACK according to a result of processing the PDUs. In general, the UE stores the PDUs in a soft buffer or discards them based on a status of a previous transmission. The UE decodes the stored PDUs and performs scheduling for multiple decoded PDUs received from several transmissions. According to a Medium Access Control (MAC) specification, 3GPP TS 25.321 V7.0.0, developed by the 3rd Generation partnership Project (3GPP), a PDU has a transmission sequence number (TSN) for scheduling use, which has a value of 0-63. Moreover, the specification defines a new data indicator (NDI), having a value of 1 or 0, for indicating that a transmission is a new transmission or a retransmission with respect to the previous transmission. Using the NDI, the UE can determine if there is new data included in the PDUs. Every time a new transmission is being established, such as after receiving an ACK from the UE, the UTRAN increments the previous NDI value by one and then sends the updated NDI for the new transmission. On the contrary, when establishing a retransmission, such as receiving a NACK from the UE, the UTRAN uses the same NDI. Thus, if the UE receives NDIs having different values, such as ‘1’, ‘0’ or ‘0’,‘1’, in two consecutive transmissions, the latter transmission is regarded as a new transmission with respect to the former one. The UE then decodes the PDUs of the latter transmission and stores the PDUs in the soft buffer to replace originally existing data. Oppositely, if the UE receives the NDIs having the same values, such as ‘1’, ‘1’ or ‘0’, ‘0’, in the two consecutive transmissions, the latter transmission is regarded as a retransmission with respect to the former one. In such a situation, if the former transmission is known decoded unsuccessfully, the UE performs soft combining on the PDUs of the latter transmission and the data in the soft buffer. Meanwhile, the data in the soft buffer may be the PDUs of the past several transmissions due to unsuccessful decoding. Oppositely, if the former transmission is known decoded successfully, the UE transmits the data in the soft buffer to a scheduling unit and feeds back an ACK. However, the PDUs of the latter transmission are discarded by the UE. If a certain transmission error occurs, such as discontinuous transmission (DTX), the HARQ process may cause the UE to miss useful PDUs, resulting in incomplete collection of data blocks in the upper layer (RLC). One following example explains such an erroneous situation in detail.
Please refer to FIG. 1, which is a schematic diagram of an HARQ process for transmitting packets in HSDPA according to the prior art. In FIG. 1, assume that a UTRAN (network end) prepares four PDUs for a UE and sends corresponding NDIs for each transmission. The TSNs of the PDUs are 8, 12, 18, and 24, respectively. In the beginning, the UTRAN transmits the PDU with NDI=0 and TSN=8. The UE stores the PDU in a soft buffer after receiving and decoding the PDU successfully, and thereby feeds back an ACK to the UTRAN. Due to the ACK reported, the UTRAN then transmits the PDU with NDI=1 and TSN=12 to the UE. Meanwhile, a transmission error occurs such that the PDU is lost during transmission and the UTRAN receives an ACK. However, the UE does not detect the transmission (the PDU with NDI=1 and TSN=12) and does not transmit the ACK, either. In this situation, the UTRAN mistakenly determines that the UE successfully receives the PDU with NDI=1 and TSN=12, and subsequently transmits the PDU with NDI=0 and TSN=18. From the standpoint of the UE, the PDUs with TSN=8 and TSN=18 have the same NDI value, so the PDU with TSN=18 is considered a retransmission of the PDU with TSN=8. According to the foregoing MAC specification, 3GPP TS 25.321 V7.0.0-11.6.2.2, the UE will transfer the data in the soft buffer, which includes the PDU with TSN=8, to the scheduling unit due to successful decoding of the PDU with TSN=8. The PDU with NDI=0 and TSN=18, mistakenly determined to be a retransmission packet, is discarded by the UE. The UE misses the PDU, including new data, such that the upper layer cannot completely collect and reconstruct the data blocks, resulting in radio resource waste.